Lockheed Electra 10E Special - NR16020

Joe Gurr: "The airplane, a Lockheed No. NR 16020, was really fascinating. It was obviously engineered and equipped for very long non-stop flights. The fuselage was solid gasoline tanks, two rows of them, from bulkhead to bulkhead, with just enough room between the tops of the tanks and the overhead for a person to be able to crawl from the stern to the cockpit on their tummy. Located in the stern, just opposite the main entrance door was a small table for the navigator's use. The radio transmitter was installed above this table. Jammed in what space they could find was a toilet."[1]

"Gurr's recollections do not correspond exactly to what is known about the aircraft: there was just one row of tanks, the transmitter was below the table, and the toilet was in a standard location."[2]

Lockheed Electra 10E Special at Burbank, California on May 21, 1937 just prior to the departure of the second world flight attempt. Noonan can be seen beside the open trunk of his Terraplane roadster. His wife, Mary Bea, stands near the front of the car. Amelia is talking with someone near the cabin door of the airplane.

Powerplants

"Lockheed had introduced the Model 10A Electra in 1934. Powered by two Pratt & Whitney Wasp Jr. SB engines of 450 h.p., the type enjoyed widespread success as a ten-passenger airliner. Deliveries of the 10E variant, featuring the more powerful 550 h.p. Wasp S3H1, began in January 1936. Earhart’s was the fifth airframe so equipped." [3]

"There's no such thing as a 'Wasp Senior.' The P&W R1340 was known as the 'Wasp.' The Model 10A Electra carried the smaller R985 which was dubbed the 'Wasp Junior' but nobody ever referred to the big engine as a 'Wasp Senior.' All Model 10Es were built with P&W R1340 S3H1 Wasp engines and NR16020 was wearing the same engines it was built with when it disappeared."[4]

Data for Dec '35 and May '41 are from P&W power curves for the S3H1/R-1340-AN-1 engines. The remaining three dates represent information taken from P&W data sheets for the S3H1 engine. With the advent of higher octane fuel, Pratt & Whitney was able to increase rated power from 550 to 600 horsepower (with one temporary excursion to 610 horsepower).

One way higher octane fuel can increase fuel economy is by changing the engine compression ratio. Efficiency increases somewhat by raising the compression ratio. This change was not employed on Amelia's engines, however. The ratio on her engines was 6:1 and remained at this level because she was using 80 octane fuel. As a matter of historical interest, Pratt & Whitney maintained this compression ratio during subsequent years even though high octane fuels became available.

Cruise economy is gained by keeping the manifold pressure up and the rpm low for a desired engine horsepower output. The piston engine is basically an air pump. For instance, Kelly Johnson recommended that Amelia run a fuel/air mixture ratio of 0.072 for all but one cruise power setting. At this ratio, 13.9 pounds of air were mixed with every pound of fuel consumed by each of Amelia's two Wasp engines.

Windows come and go

"As delivered in July 1936, the airplane had only two windows in the cabin. These were the aftmost standard airline windows and were directly opposite each other. Then in January of 1937 a window was installed in the cabin door on the port side and a larger-than-standard window was installed on the starboard side. This last window is the one that was later skinned over in Miami."[5]

Normal cruise speed

"If Earhart followed Kelly Johnson's recommendations (and there's no way to know whether she did) she reached the LOP with about 190 gallons of gas left or about 5 hour's flying time at 38 gph - but that was at 10,000 feet pulling 24 inches manifold pressure at 1,600 RPM which delivered a true airspeed of 130 kts.

"At last report, Earhart was flying at 1,000 feet presumably to get below he scattered cloud deck to look for Howland. If she wanted to keep her speed up at the low altitude he would have to bump up her power setting and therefore her fuel consumption. If she wanted to minimize her fuel consumption she would have to accept a lower airspeed.

"What did she do? How far did she run north on the LOP before turning around and running south? Did she climb or did she stay low? How much power was she carrying. How fast was she going? What was her fuel burn? Nobody knows."

Recommended power and fuel management

"Report Number 487" from Lockheed Aircraft Corporation gives various power curves for the 10E special. This report is dated 19 June 1936, roughly eight months prior to the [[first attempt|Earhart's first attempt to fly around the world,] which began on 17 March 1937.

"In the final days of preparations for Amelia Earhart’s first world flight attempt, Lockheed engineer Clarence 'Kelly' Johnson sent three telegrams in which he discussed the power management procedures which he recommended that Earhart follow to obtain the best efficiency on her flight from Oakland, California to Honolulu, Territory of Hawaii. [The telegrams are from 11 and 13 March 1937.] Johnson arrived at his recommendations through actual test flights with Amelia in her own airplane. The numbers were checked and confirmed by A. H. Marshall at Pratt & Whitney Aircraft in Hartford, Connecticut. For the serious student of the Earhart disappearance, these documents provide valuable insight into the endurance capabilites of NR16020" (TIGHAR Tracks 17:2 [2001] 6).

In the first telegram he doesn't address the initial climb and starts off recommending 3 hours at 4,000 feet carrying 1800 RPM and 28 inches of manifold pressure with a Cambridge setting of 073 which is supposed to yield 58 gph. Then, for the next three hours, he wants her to climb to 6,000 and back the props off to 1700 RPM and pull the power back to 26.5 inches with a Cambridge setting of 072 which will yield 49 gph. It's only after six hours into the flight that he has her at 8,000 feet leaving the props at 1700 RPM and pulling the power back further to 25 inches with the same 072 Cambridge setting for an estimated 43 gph.

In the third telegram he appears to abandon this recommendation for a "stepped" six hour climb to 8,000 and instead recommend a continuous climb at a honkin' 2,050 RPM and 28.5 inches with a Cambridge setting of 078. He doesn't mention what the fuel consumption will be at these settings but it's probably on the order of 100 gph. The idea, apparently, is to get up to an efficient altitude as quickly as possible. Once there, he then recommends three hours at 1900 RPM and 28 inches with the Cambridge set at 073 for an expected 60 gph. The next three hours are 1800 RPM and 26.5 inches with the Cambridge at 072 for an expected 51 gph.

Without a good idea of how long it would take to climb to 4,000 in the first instance and all the way up to 8,000 in the final recommendation, it's hard to say just what the total anticipated fuel burn would be in each case, but it does look like the final recommendations are easier on the engines (less time at high manifold pressures and relatively low RPM).

The Lae takeoff was almost certainly followed by a relatively short (10 minutes?) period of time flying in ground effect with everything to the wall before the airplane was able to climb at all and the power could be reduced.

900 gallons fuel ample for 40% excess range to Honolulu for conditions given in wire this morning.

If necessary, mixture can be leaned to 070 on last half of flight if exceptional head winds exist.

Check spark plugs before takeoff.

Hold altitude given in wire within 2000 feet if winds under 10 MPH are encountered.

Battery System

Most of the airplane's electrical needs were met by 12-volt batteries. A dynamotor (a DC motor-generator or "dynamo" driven by 12-volt current) supplied high voltage to some of the transmitter's circuits.

Each battery could be switched onto or off the main electrical bus. We assume both batteries were fully charged on arrival at Niku, and that Earhart kept one on the bus, with the other off-bus as a safety standby.

The generator was an Eclipse type E-5, rated to provide 15 volts DC, mounted on the starboard (right) engine. A crucial question for our analysis was whether the E-5 would deliver 15 volts when the engine was idling at 900 RPM. Paul Mantz, Earhart's technical advisor, said in 1937 that the Pratt & Whitney Wasp S3H1 engine--the type used on the Electra--burned 6 gallons per hour at 900 RPM. TIGHAR verified the engine and generator performance in 2009, in an experiment using an S3H1 engine and an E-5 generator. This result established that 900 RPM was the lowest speed Earhart could use for battery charging, and that she would burn 6 gph while doing so.

The time required to recharge the battery after a transmission period, and the associated engine fuel burn, are important considerations in the post-loss signals.

Ideally, we would have the Electra battery charging specifications, but we have been unable to find that information. We could not use a modern 85 AH battery as a proxy because we could not be sure that the the internal resistance and charging efficiency of the 1937 battery were the same as in a modern battery. Therefore, it was necessary to derive the battery's performance parameters from first principles, using published empirical research data for aircraft lead-acid batteries of the period.

Aircraft lead-acid battery cells of the period had a specific gravity range of 1.300 at full charge to 1.110 at fully discharged, with corresponding voltage range of 12.86 volts to 11.81 volts.

The transmission system drew 65 amps at 12 volts, and thus could function at virtually full output even with the battery approaching zero charge. Hence, the combined available "end game" battery charge--after all engine fuel was exhausted--was 170 AH, if both batteries were fully charged at fuel exhaustion.

The transmitter operated from the 12-volt DC electrical system aboard the aircraft. The tube filaments and the relays in the control circuitry were powered directly from 12 volts. High-voltage power for the tubes was provided by a dynamotor, a motor-generator unit which operated from the 12-volt system and produced 1050 volts DC at approximately 300 milliamperes.

Primary power requirements for the transmitter were approximately 11 amperes[8] on standby (tube filaments alone), and 65 amperes on transmit using voice (tube filaments, relays, and dynamotor). The tube filaments were energized continuously in standby mode; instant-heat tube technology had yet to be developed. The dynamotor operated, in voice mode, when the press-to-talk circuit was activated using the microphone button. When the transmitter was switched to C-W mode, the dynamotor ran continuously.

The transmitter was not powered by 1050 volts, per se. The transmitter received 1050 volts (at pins 13 and 16) from the dynamotor when it was activated for transmitting. This voltage was applied to the plates of the three tubes comprising the 1st and 2nd RF amplifiers. It was dropped across series resistors R10.1 and R10.2 to 380 V and applied to the plates of the oscillator and audio amplifier tubes (one each). That's all that 1050 volts was used for.

The transmitter received 12 volts from the battery for the tube filaments (pins 3 and 4 for +12 V and pins 1 and 2 for -12 V, or ground)--regardless of whether the transmitter was in standby (in which case it drew ~ 6 A), or in transmit (for which the average draw was 10.6 A). The transmitter received 12 volts from the battery on pin 5 for relays S4, S5, and S6, and for the oscillator crystal heaters if they were employed. Again, for both standby and transmit.

H. K. Morgan's Aircraft Radio and Electrical Equipment (1939), p. 144

13C and 13CB Western Electric Transmitters

Power supply voltage: 12 volts

Power consumption--standby: 6 amperes, approximately

Power consumption--modulated: 65 amperes, approximately

The credible post-loss signals occurred in clusters, or blocks. We assume that Earhart kept the engine running to operate the generator continuously during each block, rather than start and stop the engine for each transmission. We have developed a computer model that does the time line bookkeeping, giving the battery state of charge (SOC)--and fuel remaining--after each credible signal, and after a user-specified recharging period following each signal block. After we have identified all the credible post-loss signals, we'll plug them in to the model and see how battery charge and fuel consumption behaved over time.

Of course, the amount of fuel Earhart had on arrival at Niku depends on when she landed. We don't know exactly when that was, but we know the latest possible arrival time, which was constrained by tide depth on the reef. The maximum safe water depth for landing the Elctra was 6 inches (0.15 meter). Signal propagation analysis for the last Earhart signal heard by the Coast Guard cutter Itasca on July 2, 1937, gives us a bound on Earhart's distance from Niku then, and thus her earliest possible arrival time. These two limits bound the uncertainty of fuel remaining on arrival, and factor into our analysis.

The tide level also constrained when Earhart could run the engine for battery charging. The required propeller tip clearance was 24 inches (0.6 meter), so engine operation was impossible when the water level exceeded that limit.

Now, let's take a brief look at the dynamics of the electrical load and battery recharge.

The generator output current was regulator-limited to 50 amps.

We assume an ambient current load of 8 amps during each signal block: radio receiver on (1 amp), transmitter in standby (6 amps for vacuum tube filaments), and the cockpit instrument lights (2 amps).[9]

The transmitter drew 65 amps when transmitting, raising the total current load to 68 amps.

With the transmitter in standby, the generator could supply the 8 amp ambient load and have current to spare for charging the battery. But when the transmitter was keyed, the generator could supply only 50 of the required 68 amps. The battery would supply the remaining 18 amps, losing 18 ampere-minutes (0.3 AH) of charge for each minute of transmission time. About 13 minutes of charging time was needed to restore charge for each minute of transmission time, if the starting battery SOC was near the top of the exponential charging curve. At lower starting SOC, where the curve is steeper, the recharge time was on the order of 2 minutes for each minute of transmission.

Earhart's only way to monitor battery SOC was to watch the generator current output on the ammeter. The current would decrease toward the 8-amp ambient load as the battery approached full charge. But it could take a long time to get those last few ampere-hours into the battery if she wanted to recharge to 100%, and Earhart would have to be careful not to spend her precious fuel too lavishly. We don't know what strategy she used, but we can experimentally investigate the range of options and their effects using the computer model mentioned above.

Buoyancy of NR16020

"The opinon of supposed experts at the time was that, with all those empty fuel tanks, the Electra would float 'indefinitely.' We actually had some calculations run by Oceaneering International in 1991. There were 12 individual fuel tanks aboard NR16020 – three in each wing and six in the cabin. If all the tanks were empty and intact, the 7,000 lb (empty weight) airplane would be 1,200 pounds buoyant. Damage to one, or even all, of the tanks in one wing should not be sufficient to sink the airplane."[10]

References

↑This 100 gph rate of fuel consumption is not in the Kelly Johnson telegrams. "The Johnson telegrams are admittedly awkward to decipher ... The telegrams do not make specific reference to 100 gallons per hour. That number comes from a 1988 article in Lockheed Horizons, an internal company publication, by editor Roy A. Blay" (Ric Gillespie, Forum, 9 Sepember 2000).

↑Brandenburg and others say that the transmitter only drew 6 amps on standby.

↑These numbers don't add up. It seems that the 8, the 6, or the 2 must be in error. MXM, SJ.